corneal topography of humans with congenital ...

Ophthal. Physiol. Opt.,Vol.4, No. l, pp. 3 l3' 1984 Printed in Great Britain

0215-5408/84$3.00+ 0.00 PergamonPressLtd Ct 1984BritishCollegeof OphthalmicOpticians(Optomelrists)

CORNEAL TOPOGRAPHYOF HUMANS WITH CONGENITAL NYSTAGMUS C. M. DtcrINsoN and R. V. AnaoI* ..

M60 lQD, U.K. Departmentof ophthalmicoptics, UMIST, PO Box 88, Manchester (Received23 Februory 1983' in revisedform 6 April 1983)

Abstract-The refractivestatusof individualswith congenitalnystagmus(albinosand idiopaths)has been examined.Our resultsindicatethat there is a greaterthan normal incidenceof high spectacle astigmatism.This is predominantlywith-the-ruleand corneal in origin (anterior surface)' Partial residualastigmatismis frequentand doesnot differ significantly neuiralizationby the against-the-rule populations. Topographicalstudiesof the anteriorcornealsurfacerevealnormal for from that reported ed that there was no statisticaiysignificantdifferencein the mean shapefactor valuesfor the albino' idiopath and control populations.The aetiologyof the astigmatismis discussed'

INTRODUCTION The majority of those individuals who are diagnosedas havingcongenitalnystagmus(CN) fall into two main aetiologicalgroups: the albinosand the idiopaths. As long ago as l9l I Pearson et al. had reported that albinos have a tendencyto have high refractiveerrors, with myopia greatly in preponderanceand high degreesof astigmatism common.Indeed,in their study of 285 albinos, only 3.590 had astigmaticerrors of less than 1.0 D. Further confirmationof theserefractive trends has come from Fonda (1962) who reported that ll7 out of the 294 albino eyes examined in his survey showed levels of astigmatismof 3.0 D or greater. It is also of interestingto note that thesehigh incidences considerabledegreesof astigmatismappear to be independentof the racial backgroundof the individual, sincesimilar trendshave also been observedin the albino populationof the Caribe Cuna Indians of Panama (Courtney and Keeler, 1964)and in Scottishalbinos(Taylor, 1978). A significant astigmatic component also appearsto be often presentin the refractive correction of those individuals who are *Fellow of the British College of Ophthalmic Opticians (Optometrists). To whom correspondence should be addressed.

classifiedas idiopaths.Gamble's(1934)stud.y of 65 children with nystagmusrevealedthat 24Voof all eyeshad 2.0 D or more of with-therule (WTR) astigmatism,while Norn (1964) claimedthat, in his group of 140eyes,1390had astigmaticerrorsin excessof 2.0 D. In comparison,althoughit seemspossibleto detectsome degreeof astigmatismin 9590 of the normal population,typicallyabout 65Voof casesshow lessthan 1.0 D of astigmatismand 9090lessthan 2.0 D (Duke-Elderand Abrams, 1970;Borish, 1975).Furthermoremany studies that curvatureastigmatismof have established of the corneais responsible surface the anterior for the vast majority of cases of regular astigmatism(Duke-Elder and Abrams, 1970; Borish, 1975;Katz, l98l). This usually takes the form of WTR astigmatismwherethe more powerful meridian is vertical (90 + 30'). Although some astigmatism has also been ascribedto the posteriorcornealsurface,it is generally consideredto be far smaller and (ATR), with the more powerful against-the-rule mefidianhorizontal(180+ 30'). Other possible sourcesof astigmatisminclude the crystalline lenscurvatureand lenstilting. Any astigmatism which originatesfrom a siteother than from the enterior corneal surface is conventionally referredto as residualastigmatism. ln this presentinvestigationour aim is to

C. M. Dickinsonand R. V. Abadi

examinethe refractive status of two groups of nystagmats:the albinos and the idiopaths. The data will be correlatedwith the measurements of the curvature of the anterior surface of the cornea,to establishto what degreethe anterior cornea contributes to the total amount of astigmatism. Rather than just rely on conventionalkeratometric measuresof corneal curvature, which give indications of curvatures only within a small annulus of the central cornea, we decided that a more meaningful quantitative index would be to record the topography of the whole anterior corneaand so provide a measure of the rate of peripheral flattening of the cornea. SUBJECTS A total of 54 subjects with CN aged from 6 60 years were examined in this present study. They were classified into two groups -albinos (23) and idiopaths (31). The albino population included oculocutaneous(T* and T-) and ocular albinos. Our subjects did not represent a random cross-section,since they were either referred by local ophthalmic opticians and ophthalmologistsor volunteeredby personal application. None of the 54 subjects had a history of contact lens wear or ocular surgery. Although albinos and idiopaths make up the greatest proportion of those with CN, there does exist a third identifiable aetiological group. Those individuals who fall within this category have a congenital ocular pathology such as cataracts, optic atrophy or buphthalmos. This group were deliberately excluded from this study becausethey are often found to have undergoneeye surgeryinvolving corneal sectionwhich could well affect or contaminate the experimentalfindings. A control group of 13 normal subjectsmatched for age was chosen.They had no history of seriousocular or systemicdisease.It was felt necessaryto have a control group sinceit seemed unrealistic to compare our regrettably limited sampleof nystagmats(dueto the low incidenceof the condition) solelywith the existing data; in previous surveys of refractive com-

ponents and corneal topography the samples have tended to be very large (i.e. n > 1000). Controls with high valuesof astigmatismwere deliberately chosen so as to determine if any relationship existed between the shape factor and large WTR and ATR astigmaticrefractive errors. At each stage of the study, the availability and suitability of our subjectsdictated the number of separateeyesthat were examined. METHODS All subjects were refracted by retinoscopy and conventionalsubjectivetechniques. Preliminary examination of the corneas by keratometry (Sbisa fixed doubling instrument) revealed no overt corneal irregularities. The mire images were undistorted and the principal meridiansorthogonal (i.e. regular astigmatism). Although this was a simple and quick clinical techniqueit was far from optimal for curvature quantification. This is because measurementof a given meridian only usestwo small areasastride the visual axesand assumes a sphericalcorneawith the peripheryfollowing the shape of the central area. The size and separationof theseareasalso variesbetweeninstruments and with the corneal radius being measured(Mandell, l98l). Moreover making measurementsof corneal curvature on a continuously moving eyewould also meanthat different areaswould be sampled. For a more quantitative assessmentthe Wesley- Jessenphoto-electrickeratoscope(PEK) System 2000 was used to determine not only the central curvature but also the whole corneal contour (Fig. 1). The keratoscope target is a seriesof sevenconcentricrings on an ellipsoid.surfaceand its reflectedcornealimage is photo$raphedon a Polaroid Land Projection Film (Type 146-L;high contrast).From a x 50 enlargementof this image the cornea is three dimensionallymappedby computer analysisto give the topographicaldetailsof the whole cornea (Bibby, 1976;Bibby and Townsley,1976a, b). The difficulty in using this instrument on nystagmatsis in ensuringaccuratefixation. The

Corneal topography of humans with congenital nystagmus

Mi*$"

Fig. l. The Wesley-Jessen photo-electronic keratoscope (top) (below)'

PEK, however,incorporatestwo devices,one for useby the subjectand one for the observer to aid accuratealignment.Observationof the final photograph will indicate whether the reflectionis well centredwithin the visible iris diameter or is badly posi-tionedand partially reflectedfrom the bulbar conjunctiva. The PEK was testedto measurethe radiusof a knownsphereto within + 0.015mm of its true

typical keratograph

value. Reproducibility studies indicate a maximum SD of fr D in the radiusof the steel ball (Bibby and Townsley,1976a). Describing corneal shope Many studieshave now establishedthat the contour of the anterior surfaceof the cornea can bestbe describedas an ellipsoid(Townsley, 1970; Mandell and St. Helen, l97l; Bibby,

C. M. Dickinson and R. V. Abadi

1976), since normal sections through the ellipsoidparallel to the co-ordinateplanes(in the presentlimiting case,throughthe astigmatic meridians)are elliptical and therefore along with the circle, parabola and hyperbola are conic sections.In order to adequatelydescribe such conic sectionstwo parametersneedto be ascertained. (l) Central curvature,which is the curvature at the apex of the curve. This simply defines sizebut is independentof shape. (2) Shape factor (SF), which definesshape but is independentof size.Computationof the SF will thereforeindicatethe peripheralshape of the cornea and how the corneal curve changesfrom centreto periphery.NegativeSFs indicatethat the curve follows that of the flat (oblate) side of the ellipse whilst positive coefficientsimply that the curve follows the more pointed(prolate)end of an ellipse.An SF of zero would mean that the meridian being describedis exactly an arc of a circle. The higher the numericalvalue of the SF the more ellipticalis the shapeof the cornea.Thus a low positive SF indicatesa low rate of flattening and a high positiveSF a high flatteningrate.On the other hand a negativecoefficientindicates peripheralsteepening.In terms of traditional Euclidiangeometrythe lSFlt is the eccentricity of the ellipse. Means (I) and SDs were computed in the customarymanner.The differencebetweenthe meansfor cornealcurvatureand SF in different subject groups were tested for statistical significanceusing Student'st-test(two-tailed).

meridians for the left eyesof each albino and eachidiopathic nystagmatare illustratedin Figs 2 and 3 respectively.In both plots it can be seen that there is a significant weighting towards WTR astigmatismwith 6790 of the albino and 3390of the idiopathsshowing2.0 D or more. We thereforeconfirm that thereis indeeda high incidence of astigmatism to be found in individualswith CN.

The curvature of the central cornea Histogramsshowingthe distribution of corneal curvature for both principal meridiansare illustrated in Fig. 4. "Horizontal" and "vertical" should not be taken literally as these terms are used only to indicate that the meridian depictedis the one most nearly horizontal or most nearly vertical. For the albino group there is a statisticallysignificant differencebetweenthe horizontal(; = 8.01 + 0.43 mm) and the vertical(x : 7.51 -r 0.46 mm) meridiansat the P < 0.001level.That is, thereis lessthan a 0.190probabilityof thesetwo meansbelonging to the same population. The idiopaths also show a significant difference between their horizontal(x : 7 .99+ 0.36mm) and vertical(x : 7.75 + 0.25 mm) meancentralcurvatures(P = 0.001). However, although there was a significantdifference(P = 0.01) betweenthe vertical meridian means for the albinos and idiopaths, the differencebetweenthe meansfor the horizontal meridian was not signifcant (P > 0.8). The normal control group werenot included in the comparative analysis of corneal curvature as they were deliberately selected specificallyfor their large astigmaticrefractive RESULTS errors and they therefore would not reflect a There are four indices by which the non-biasednormal samplepopulation. Conseastigmatismof normal and nystagmateyescan, quently our data shouldbe comparedwith that be compared: (l) the type and degree of of Townsley(1970)who examined360 normal refractivecorrection,(2) the curvatureof the corneas with the PEK and found the mean central region of the anterior surface of the value for the combined horizontal and vertical cornea, (3) the shapeof the peripheryof the central curvatures to be 7.70 + 0.25 mm. anterior surface of the cornea, and (4) the Although we have specifiedthat the PEK was amount of residualastigmatism.We will now our instrumentof choicewe alsomeasuredcendescribethe resultsfor eachsectionseparately. tral corneal curvature with the Sbisa The type and degreeof refroctive correction keratometerto seeif the two instrumentsfurThe refractive powers of the two principal nishedequivalentreadingson the samepatient.

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C. M. Dickinsonand R. V. Abadi Vertical

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With the normal controls who did not have nystagmus,this was indeed the case.However for the subjects with nystagmus there were numerous occasions when there was a data mismatch(> 0.05 mm). Two reasonsare proposed for this. First, the central curvature reading from the PEK is measuredacrossthe inner ring of the keratographand correspondsto a 3-mm chord of the cornea.This may not exactlycorrespond to the dimension of the chord monitored with the Sbisakeratometer.Moreover the separation of the two areas in each meridian varies as a function of the cornealradius of curvatureand the dimensions and the position of the keratometer mire targets. Second, the keratometer reading is probably "averaged" over many chords of the cornea due to the constant nystagmus.This presumably means that the mire images are in constant oscillation towards and away from each other as areasof different curvatureare sampled.The clinician is

not aware of this oscillation and the constant averagingof the reading.This presumablydoes not occur with the PEK where our alignment procedureand criteria for an acceptablepicture (clear undistorted negative with at least four complete rings imaged) suggestthat we were sampling a fixed chord on the cornea. Over the yearstherehavebeenseveralreports which have describeddiurnal changesin corneal curvatures.The cornea it seemsis flattest upon awakening and then steepensduring the day (Reynoldsand Poynter, 1969;Kiely et al., 1982).Unfortunately becauseall our subjects volunteered their servicesfor this project we were unable to arrange that the curvature readingswere taken at exactlythe sametime of day, though the majority of readings were taken after midday. The documentedmaximum daytime changesin mean horizontal curvatures of 0.05mm and meanverticalcurvatureof 0.03 rilrn or€ too small to affect the significanceof our results.

Corneal topography of humans with congenital nystagmus

The shape of the periphery (Bibby, 1976)and does not significantly differ The frequency distribution of SFs for the from that measuredin individuals with CN. normal control group, the albinos and the The amount of residual astigmatism idiopaths are shown in Figs 5 and 6. The former The calculated residual astigmatism (CRA) figure illustrates the data for the horizontal was determined by subtracting the difference meridian and the latter for the vertical meridian- Means and SDs for the six populations betweenthe central curvature readingsof the are given adjacent to the histograms. A Stu- two principal meridians from the total dent's /-test revealedno statisticallysignificant astigmatismfound by retinoscopy.Table I lists difference between the means of the popula- the degree of CRA for the normal control, tions (P > 0.8). However observation of the albino and idiopath subjects. For all three groups ATR residual astigmatism was by far histogramssuggestthat there are differencesin the shapeof the distributions. Indeed, for the the most common. Theseresultsare similar to horizontal meridian, the normal and idiopath thosedescribedby Carter (1963).Using refrachistogramsare positively skewed(0.27 and0.72 tive error measurements and keratometer respectively)while the albino one is negatively readingshe found the CRA for a group of 100 skewed(-0.48). In the vertical meridian the eyeshad a mean value of 0.60 + 0.41 D ATR frequency distributions are all negatively astigmatism. Although contact lens practitionersprefer to skewed-normals (-0.49), albinos (-0.91) and idiopaths (- 1.76). The SF found in the define residualastigmatismas the refractiveernormal population has a mean of 0.25 + 0.18 ror that is presentwhen a contact lens is placed upon the cornea to correct the existing ametropia (Mandell, l98l) there appearsto be i : O . 27 i O . 1 1 10 little difference in the mean values of ATR NORMALS astigmatismdeterminedby this method. Sarver (1969) reports a mean of 0.50 -r 0.45 and Dellande(1970)a mean of 0.51. DISCUSSION 10

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Fig. 5. Frequencydistributions of the shape factors for the horizontal corneal meridian in normal, albino and idiopath subjects. No significant difference was found between the means for any of the three populations. See text for details.

The results of this study clearly confirm previous reports that there is indeed a greater than normal incidence of high spectacle astigmatismin individualswho haveCN. Using the PEK we have found that the astigmatismis predominantly WTR and corneal in origin. Partial neutralization by the ATR residual astigmatismis frequent and doesnot appearto differ significantly to that reported for normal populations. It therefore seemsthat the refractive errors seen in individuals with CN are amenableto the sametechniquesof opticalcorrection (e.g. spectaclesand contact lenses)as a subjectwithout CN and just as likely to be optimally improved. Corneal chongeswith oge The variation of regular astigmatism with (Hirsch, 1959;Anstice, ageis well-documented

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l97l; Duke-Elderand Abrams, 1970;Woodruff, l97l; Borish, 1975;Ingramsand Barr, 1979; Saunders,l98l; Baldwin and Mills, l98l). In the normal population marked astigmatism is commonly found in young infants, though it is generally agreed that the astigmatism is not present shortly after birth but increasesand becomes maximal between I I

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and 40 weeksof age (Mohindra et al., 1978). Though recent reports state that more than 45Vo of normal infants have astigmatism greater than 1.0 D (Mohindra et al., 1978; Howland et al., 1978)Fulton and her colleagues (Fulton et al., 1980)claim that this figure is reducedto an incidenceof 190/oif the refraction is carried out with a cycloplegic.This is still

ll

Corneal topography of humans with congenital nystagmus

however at least twice the adult incidence of astigmatism. Unfortunately there is still some disagreement as to the relative distribution of the astigmatismin infants. Fulton et al. (1980) quote incidencesof TlVo ATR, 219oWTR and 890 oblique, whereasMohindra et al. (1978) suggestfigures of 4090 ATR, 40Vo WTR and 2090 oblique in their ll-20 months of age samplegroup. Though it is temptingto suggest that infantile astigmatismis more likely to persist into adult life in nystagmats,there is no evidence for these high incidences of ATR astigmatismin our subjects. Photorefractive measurementssuggestthat the incidence of significant levels of astigmatism falls rapidly during the second year of life and is down to adult levelsby-about 18 months(Atkinson et al., 1980).This iendssupport to the earlier experimental findings of Hirsch (1963)who saidthat9SVoof the children he examined in his longitudinal study of children from the age of 6 to 12 years showed no more than a 0.37-Dchangeduring the 6-year period. He thus concluded that children who are going to have marked astigmatismwill have it when they enter school.There also seemsto be a generalconsensusthat, within the adult population (20-40 years), 9090 show astigmatic errors less than 2.0 D with around 9090 of these exhibiting a WTR astigmatism (Duke-Elder and Abrams, 1970). Tracing the changesin astigmatism among adults and the elderly, Hirsch (1959)reported that meanastigmatismchangedin his sampleof 800 patientsfrom 0.25 D WTR at the age of 40 to 0.75ATR at the ageof 80. Baldwinand Mills (1981) felt that, since their study on patients

of the cornea aged30- 70 indicateda steepening in the horizontal meridian, it is this change which is responsiblefor a major portion of the increasein ATR total astigmatismamongolder patients. The agedistributionsof our subjectswith CN are shown in Table 2. Of the 11 subjectsolder than 40, six were albinos and five idiopaths. Five of the six albinos exhibited WTR astigmatism,which appearsto be greater than the percentagefound in normal populations. The aetiology of astigmatism in CN Though there is sound evidenceto suggest that both corneal refractive power and corneal astigmatism have a strong hereditary basis (Wixson, 1958, 1965; Francois, l96l), environmental factors (e.g. visual experienceand diet) must also figure largely (Grosvenor, 1978).Nevertheless the factors responsiblefor the high incidenceand subsequentlability of astigmatismduring infancystill remainobscure. Lest it be forgotten, at birth the sagittal diameterof the eyeis approximatelyl8 mm and by the age of 3 years the anteroposterioraxis has extended to around 23 mm in length. Theoreticallythis elongation should produce a drift towardsmyopia of some 15.0D if it were not for compensatorychangesin the dioptric apparatus that would seem to be, at least in part, dependentfor its operation on adequate visual inputs (experience).The engineeringof the various dioptric elementsto accomplishthe regulation of an astigmatichyperopic eye(12 weeks after birth) to a near-emmetropicstate (by 3 years) has been called emmetropization. Presumablysomevisual feedbackabout image quality must be required. However it may be

Table 2. The age distribution of the subjectsexamined in this study

Age (years) Total No. of subjects